The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
The security market requirements for state-of-the-art mobile and portal radiography inspection system include high imaging resolution (better than 5 mm line pair), penetration beyond 300 mm steel equivalent, material discrimination (preferably at least three groups of Z) with preferably 100% image sampling at speeds up to or greater than 16 km/h, low dose and small radiation exclusion zone.
X-ray security systems for the inspection of cargo 20 and shipping containers typically use transmission radiographic techniques based on the use of an X-ray fan beam 116 generated by a pulsed high-energy X-ray source 110, such as a linear accelerator, linac, or betatron. FIG. 1 (prior art) depicts a cargo inspection system employing such a technique. An array of X-ray detectors 118 is processed 115 to produce an image of the cargo 190.
A fan-shaped beam of X-ray 116, emitted by a linac- or betatron-based source, is detected by elements of a detector array 118 distal to an object, here a cargo truck 20, in order to produce radiographic images of the target object 190. The particular contents of the object may be discriminated and characterized on the basis of the transmission of X-rays through the object and their detection by the detector array and its individual detector pixels. Signals from each of the detector pixels, suitably pre-processed, provide inputs to processors, where the radiographic image of the object and material characteristics are computed. The thickness of the material to be penetrated by the X-rays may exceed 400 mm of steel equivalent in some cases. To insure the required penetration, inspection systems typically use X-rays with a maximum energy of several MeV, currently up to about 9 MeV. X-rays in excess of 1 MeV are frequently referred to as high-energy X-rays.
Information (such as mass absorption coefficient, effective atomic number Zeff, electron density, etc.) about the material composition of the contents of objects may be obtained on the basis of the interaction of the X-rays with the material, and, more particularly, by illuminating the material with X-ray beams having energy spectra with more than one distinct energy endpoint (peak energy), or by employing energy discriminating detectors. Dual energy methods of material discrimination are known in the art and are widely used in X-ray inspection systems for security control of cargo in checkpoints. Dual energy and multiple energy inspection is discussed in the following references, for example, which are incorporated herein by reference: W. Neale, et al. “Material Identification using X-Rays”, U.S. Pat. No. 5,524,133 (1996), V. Novikov, et al. “Dual energy method of material recognition in high energy introscopy systems”, International Workshop on Charged Particle Linear Accelerators: Problems of Atomic Science and Technology, pp. 93-95 (1999); S. Ogorodnikov, et al. “Application of high-penetrating introscopy systems for recognition of materials”, Proceedings of EPAC 2000, Vienna, Austria, pp. 2583-2585; S. Ogorodnikov and V. Petrunin, “Processing of interlaced image in 4-10 MeV dual energy customs systems for material recognition”, In: Physical review special topics Accelerators and beams, Vol. 5, 104701 (2002), 11p; P. Bjorkholm, “Dual energy radiation scanning of objects”, International Patent application WO 2005/084352 (2005); and S. Chakhov, et al. “Betatron Application in Mobile and relocatable Inspection Systems for Freight Transport Control”, Journal of Physics: Conference Series 671 (2016) 012024, DOI: 10.1088/1742-6596/671/1/012024.
Conventional dual-energy linac- or betatron-based cargo inspection systems capable of material discrimination provide X-ray pulses with two end-point energies 201, 202 (typically 4 and 7.5 MeV, or 5 and 9 MeV) alternating from pulse to pulse with a repetition rate up to 400 pps 205 (see FIG. 2 for dual-energy interlaced betatron). In the case of the betatron-based system the values of low- and high-end-point energies are determined by the corresponding predefined extraction time intervals tL 203 and tH 204 within the betatron acceleration cycle. The typical duration of each X-ray pulse is about four microseconds 206.
A multi-energy cargo inspection system based on race-track microtron is presented in U.S. Pat. No. 8,761,335. The disadvantages of this proposed system are: that system can generate X-ray pulses with just fixed step energies, four steps system discussed in that patent; radiography performance of the inspection system based on race-track microtron is limited by beam current instabilities; the control system required for such microtron is very complicated and didn't allow adaptive control of the parameters of X-ray pulse.
The advantages of the betatron-based inspection systems over conventional linac-based designs include extended region of thickness of material discrimination validity, larger penetration, small focal spot (which improves the resolution), low weight and form-factor, smaller exclusion zone, simpler control system and relatively low cost.
However, this approach has three major limitations. First, the long time (>2 ms) between two energy pulses results in an inability to illuminate a single imaging slice with both energies in moving cargo 20. Therefore, conventional dual-energy material discrimination technique will not deliver accurate information about the regions average Z, see FIG. 3. Second, even at a repetition rate of 400 pps 205 some portion of the moving cargo is not sampled due to the detector element size and object motion effects (undersampling). Finally, this technique forces a choice of only two fixed energy levels 301, 302 which may only be suitable for material discrimination within a limited region of object areal densities. It has no ability to adapt beam energies to the real-time X-ray attenuation in cargo. This fact significantly limits material discrimination in regions of low or high densities, and unnecessarily increases the dose to the environment. For most cargoes, much lower X-ray end-point energies are sufficient to achieve cargo penetration and material discrimination.
Thus, there is a need for an adaptable inspection system able to overcome the foregoing deficiencies and provide enhanced material identification adaptable to variable characteristics of the cargo under inspection.